VCSEL structure insensitive to mobile hydrogen

ABSTRACT

An active region of a VCSEL at one (i.e., n doped) end having an expanded effectively undoped region, and another (i.e., p doped) end having a significantly doped region up to or even including a portion of the active region. A previous way had heavy doping of the n and p doped regions up to the active region, at least close to it or even partially into it.

This is a continuation of application Ser. No. 08/989,731, filed Dec.12, 1997, now U.S. Pat. No. 6,256,333.

BACKGROUND OF THE INVENTION

The present invention pertains to vertical cavity surface emittinglasers (VCSEL's) and particularly to VCSEL's made by a metal-organicchemical vapor deposition (MOCVD) process.

The perspective view shown in FIG. 1 illustrates a typical structure fora vertical cavity surface emitting laser 10. A gallium arsenidesubstrate 12 is disposed on an n type electrical contact 14. A firstmirror stack 16 and a bottom graded index region or lower spacer 18 areprogressively disposed, in layers, on the substrate 12.

Region 20 may have one or many quantum wells or may be a bulk activegain region. An active region 20, having one or more quantum wells, isformed and a top graded index region or upper spacer 22 is disposed overactive region 20. The spacers are to provide the appropriate criticaldistance between the mirrors to provide the proper-sized resonant cavityfor a given wavelength and the distance is related to that wavelength ora multiple thereof. Active region 20 has a gain that compensates for theleaking out of photons. Photons bounce back and forth and, due to),imperfect mirrors 16 and 24, eventually leak out of the device.Greater photon loss means more gain is needed.

A p type top mirror stack 24 is formed over active region 20 and a metallayer 26 forms an electrical contact. Current 21 can be caused to flowfrom the upper contact 26 to the lower contact 14. This current 21passes through the active region 20. Upward arrows in FIG. 1 illustratethe passage of light 23 through an aperture or hole 30 in the uppermetal contact 26. Downward arrows illustrate the passage of current 21downward from the upper contact 26 through p type GaAs cap layer 8, ptype conduction layer 9, p type upper mirror stack 24 and active region20. A hydrogen ion bombardment or implantation 40 forms an annularregion of electrically resistant material. In order to confine thecurrent flow 21 through active region 20, device 10 uses a hydrogen ionimplant technique to create electrically insulative regions around anelectrically conductive opening extending therethrough. A centralopening 42 of electrically conductive material remains undamaged duringthe ion implantation process. As a result, current 21 passing from uppercontact 26 to lower contact 14 is caused or forced to flow throughelectrically conductive opening 42 and is thereby selectively directedor confined to pass through a preselected portion of active region 20.

The present problem concerns active region 20 of the device. The issuerelates to the reliability implications that result from the interactionbetween carbon and hydrogen in the VCSEL structure. There have beenvertical cavity surface emitting lasers that have had short termdegradation caused by hydrogen passivation or compensation-of carbon.Hydrogen compensates carbon acceptors in AlGaAs. This phenomenon is abyproduct of the MOCVD growth process and also results from protonimplantation. Carbon ions are used in doping. Carbon doping brings in asignificant amount of hydrogen. The results of hydrogen passivation arerapid degradation of the devices sometimes followed by rapidimprovement, which is the result of the hydrogen moving through thestructure under bias. Longer baking during the fabrication processdrives out more hydrogen.

There are several kinds of doped structures. If low doping ≦5×10¹⁷/cm³(5el7) (curve 13 in FIG. 2a) is used in p-spacer 22 of FIGS. 1 and 3,then the structure is sensitive to mobile hydrogen. The sensitivity tomobile hydrogen occurs because hydrogen acts as a donor and compensatesthe carbon. However, the hydrogen is very mobile and under field-aideddiffusion, hydrogen H drifts towards active region 20 and compensatescarbon C acceptors on the edge of active region 20 in FIG. 2a. One wayto overcome this compensation is to use higher p doping near activeregion 20 (see curve 15 in FIG. 2a). The problem that arises with suchdoping is that there remains a large slope 19 in active region 20 evenat lasing voltages in FIG. 2c. The separation of the carriers resultingfrom energy band slope versus position 19 makes the recombinationinefficient. To overcome this problem, a thick (15 nanometers)effectively undoped region is placed on lower side 18 of active region20. The voltage is then allowed to drop across the undoped region.

The post growth anneal and the use of low arsine over pressures duringgrowth have been other solutions attempted to prevent hydrogenpassivation or compensation of carbon that causes at least short termdegradation of VCSEL's. The present invention is a structural solutionto the problem.

Schneider et al, U.S. Pat. No. 5,557,627, discloses a visible VCSEL thatallows the use of Carbon as a p-type dopant, which improves the dopantprecision and stability against thermal diff-usion during epitaxialgrowth and subsequent device processing. To allow the use of Carbon, abarrier or spacer layer is fabricated on the p-side of the activeregion, having little or no indium.

In Kobayashi et al, U.S. Pat. No. 5,513,202, a VCSEL having a substrate,a p-type mirror, a p-type spacer layer, an active region, an n-typespacer layer, and an n-type mirror is disclosed. The problem addressedin this patent concerns the high vertical resistance in a non-gradedp-type mirror. One embodiment disclosed involves a p-type spacer layerwith an impurity concentration higher than that of the p-type bottommirror. More specifically, Kobayashi discloses that the p-type spacerlayer has an impurity concentration of 3×10¹⁸ cm⁻³. Additionally, then-type spacer layer has a concentration of 3×10¹⁸ cm⁻³, but may also bean undoped layer.

SUMMARY OF THE INVENTION

A structural way to make the VCSEL structure 10 less sensitive to thehydrogen passivation problem is to use heavily doped layers near activeregion 20 (of FIGS. 1 and 3). These layers would be too heavily dopedfor the hydrogen to completely compensate. If this doping is notcarefully performed, device 10 will not work because energy bandstructure 17 of active region 20 will have a residual tilt 19 even atlasing voltages (as shown in FIG. 2c). This represents an electric fieldacross active region 20. The electric field causes the carriers of theopposite charge to preferentially seek one side or the other of activeregion 20 and radiative recombination becomes inefficient. Sinceradiative recombination is inefficient, parasitic recombinationmechanisms dominate.

To eliminate this residual tilt of bands 17, an effectively undopedsection 18 in the n graded region (or close to active region 20 on then-side which may include an n spacer) must be present and this undopedregion 18 must be of sufficient extent. The term “effectively undoped”means that there are residual impurities in any material and that thereis in reality no such thing as strictly undoped material. For purposesin this description and the claims, “unintentionally doped,”“effectively undoped” and “undoped” mean the same thing and may be usedhere interchangeably. Additionally, p doping can be added to activeregion 20. Two dissimilar materials (i.e., n and p doped), when placedtogether, have different work functions and charges that flow from oneto another. So before a bias voltage for lasing is applied, there is abuilt-in voltage between the p and n regions. This voltage causes anelectric field/energy band slope which is reduced by reducing the chargeat the junction. This is accomplished by introducing an undoped(uncharged) region. As voltage is applied the band flattens further andthe electric field is reduced.

In summary, the invention has two features. It provides for relativelyhigh doping in the p regions 22 down to and optionally through activeregion 20, and it has a thick undoped region in the lower graded region18. The high p-doping in spacer 22 makes the structure insensitive tohydrogen, and the thick lower undoped region 18 allows the electricfield to drop across this region making the active region energy wellsrelatively flat at lasing voltages (in FIG. 4c). An alternate structuremay be that in the above, the p doping and regions be interchanged withthe n doping and regions.

In essence, to eliminate sensitivity to mobile hydrogen, a heavily dopedregion (i.e., in the upper spacer) needs to be placed adjacent to theactive region. To reduce the tilt of the energy bands in the activeregion (under active lasing bias voltage across the VCSEL) resultingfrom the heavily doped region adjacent to the active region, an undopedor unintentionally doped region (i.e., in the lower spacer) ofsufficient extent is placed on the other end of the active region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative example of a vertical cavity surface emittinglaser.

FIGS. 2a, 2 b and 2 c show a typical doping scheme around the activeregion which is sensitive to mobile hydrogen, an aluminum contentprofile and resultant energies.

FIG. 3 is a cutaway of the various layers and the current confiningimplant.

FIGS. 4a, 4 b and 4 c reveal the doping scheme, the aluminum content,and energy band structure of the invention.

FIGS. 5, 6 and 7 show the energy band structures for various voltages,respectively, of a certain doping scheme.

FIG. 8 reveals the material composition of an example VCSEL.

DESCRIPTION OF THE SPECIFIC EMBODIMENT

The invention, as illustrated in FIGS. 4a, 4 b and 4 c, provides formoderate to heavy doping having a peak greater than 1×10¹⁸/cm³ (i.e.,3el8) in the upper p layers 22 adjacent to active region 20, that is, pdoping near active region 20; and a very lightly doped or undoped(unintentionally doped, i.e., <7el7) region 49 below active region 20,that is, the n portion of the junction adjacent to active region 20 ofat least 8 nanometers (nm) or even greater than a 10 nm extension intospacer region 18 (FIG. 4a). These levels of doping apply to VCSEL'shaving greater than a 50 percent aluminum composition in the spacerregions 18 and 22. Otherwise, the 10 nm extension would be less. It isalso beneficial to increase the hole barrier with high doping(i.e., >1el8) in the n portion more than 10 nm away from active region20 (FIG. 4b). The latter is a subsidiary feature.

The doping in active region 20 must also be either undoped or p doped,but it cannot be n doped. The p doped region can extend down into thealuminum graded region or lower spacer 18. If this extension into thealuminum graded region or lower spacer 18 is taken too much to anextreme, then a loss in injection efficiency occurs. Despite thisextension of the p region, there needs to be at least an 8 nm region ofundoped or lightly doped material below the last p layer beforesignificant n doping begins.

The preferred structure is one that has a flat active energy band regionat lasing voltages, as shown in FIG. 4c, which has large minoritycarrier barriers at the edges of active region 20, and has high dopingclose to the active region at least in the p region and optionally atlower doping levels through a portion of region 20 and even into spacer18, as indicated in FIG. 4a.

In more detail, the doping structure of VCSEL 10, as shown in FIG. 4a,has an n doped substrate 12 formed on contact 14. A first n dopedmultilayer mirror 16 is formed on substrate 12. On mirror 16 is a spacer18 as needed for forming the appropriate resonant cavity between thefirst and second mirrors. Spacer 18 is n doped, preferably at 2el8nearly up to active region 20, as shown by curve 25. The n doping leveldrops from 2el8, as indicated by line 27, to 5el7 as indicated by line29. The latter n doping level extends through spacer 18 (or mirror 16 ifthere is no spacer) up to active region 20, where it ends as line 33indicates. The length of doping level 5el7 line 29 represents typically10 to 15 nm. It must not go all the way to region 20.

Formed on spacer 18 is active region 20 layers, and on region 20 isspacer 22 or mirror 24, depending upon the need or not of spacer 22. Thep doping level may start out between 1el8 and 3el8, and increase to apeak of 3el8 at a distance of about 20 to 40 nm from active region 20,as shown by line 35. Then the p doping level drops, as indicated by line37, to 2el8 as shown by line 39. The distance represented by line 39 istypically between 10 and 15 nm. Then the doping level drops in a gradualfashion from 2el8 to 1el8 (which is the redesign) over a distancebetween 10 and 15 nm up to active region 20, as illustrated by line 41.At the place where spacer 22 is adjacent to region 20, the p dopantlevel drops to 3el7 as indicated by line 43. The p dopant extends intoactive region 20 at a level of 3el7 to spacer 18, as indicated by lines47 and 48. The p doping of region 20 is optional. Line 49 is the undopedor unintentionally doped level.

FIG. 4b shows. the percentage of aluminum content in 18 and 22, and thelayers of active. region 20. Spacer 18 has about a 60 percent content ofaluminum as indicated by line 51. Sixty percent aluminum meansAl_(0.6)Gao_(0.4)As. In the top 15 or so nm of spacer region 18, thealuminum content gradually decreases from 60 percent to about 25percent, as indicated by line 52. The gradual decreasing of the aluminumcontent from 60 percent, as represented by line 52, may start in spacer18 well before entry into region 20.

At the 25 percent level of aluminum, this content percentage may remainconstant for at least one layer of active region 20, as shown by line53. The next layer of active region 20 may have about zero percentagecontent of aluminum, as indicated by line 54. The next layer has about25 percent aluminum content as noted by line 55; the following layer hasabout zero percent aluminum as shown by line 56; and the next layer hasabout 25 percent aluminum as noted by line 57, the subsequent layer haszero percent aluminum as indicated by line 58, and the remaining layerat the top of region 20 has 25 percent of aluminum as shown by line 59.There could be more or less alternating layers with and without,respectively, aluminum, depending how many quantum wells are desired inregion 20. Of the alternating layers having aluminum content, one mayhave between twenty and thirty percent and the other between zero andfive percent of aluminum. Following this layer is spacer 22 wherein thealuminum content gradually increases from 25 percent to 60 percent forthe 15 or so nm into spacer 22, as illustrated by line 60. Line 61represents the 60 percent content of aluminum in the remaining portionof spacer 22.

FIG. 4c shows the performance aspects of the preferred structure havingan applied voltage of 1.5 volts across active region 20 of a device 10for emitting light at 850 nm. The relative tilt 19 to the active regionis not large and the barriers to minority carriers are large. FIG. 4areveals the structure having heavy doping close to active region 20.FIG. 2c illustrates an active region 20 which has moderate doping in thecarbon-doped spacer layer 22, and does not have a sufficiently low-dopedregion 20, thus resulting in a large tilt 19 in active region 20 at thelasing voltage of 1.5 volts. This large tilt 19 represents an electricfield which causes a separation of the carriers in active region 20 andcauses the radiative recombination of the carriers to be reducedrelative to parasitic currents. Also, the doping in the spacer 18 islight so there is susceptibility to hydrogen compensation.

One may note the changes at increasing bias voltages from zero to 1.5volts, as shown in FIGS. 5, 6 and 7. Note that at low voltages, nolasing occurs. These are for having an active region doped at 5el7 andthe upper spacer at 2el8. At zero volts, the active region has a largeslope across it (FIG. 5). At 1.2 volts, the active region flattens outsignificantly (FIG. 6). And at 1.5 volts, the active region issignificantly flat (FIG. 7). Doping in active region 20 flattens theenergy bands 17 at lower bias voltages more so than with no doping.Conductivity modulation increases conductivity in active region 20.Conductivity modulation in active region 20 is sufficiently high thatwhen 1.5 volts (lasing voltage) is reached, the active region energybands become flat. For the same structure of FIGS. 5, 6 and 7, exceptthe former having an undoped active region; at zero voltage, the slopeis steep; at 1.2 volts, the undoped approach still shows a significantslope remaining in the active region; and at 1.5 volts, the conductivitymodulation nearly flattens the slope in the active region. If there wasnot the wide undoped lower graded region, then a large slope wouldremain as in FIG. 2c.

An additional approach that minimizes the problem of hydrogenpassivation or compensation of carbon, is an annealing of material aftergrowth in a non-atomic hydrogen producing environment. The hydrogengradually diffuses out and the problem is reduced. A combination of theannealing and the above-noted structure of the present invention isbetter than either one alone.

FIG. 8 illustrates the composition of an example of structure 10. Inthis version, alternating epitaxial layers 45 and 46 for laser 10 aredeposited on a substrate 12 which is doped n-type. On the bottom side ofsubstrate 12 is formed a broad area contact 14 (i.e., n-ohmic). A bottommirror 16, consisting of 26 periods of alternating layers of AlAs 46 andAl_(X)Ga_((1−x))As (=0.15 is preferred, but x may have any value greaterthan 0.00) 45, all doped n-type, are grown to form a highly reflectingmirror 16. The total number of mirror periods may be greater or lessthan 26, depending on other parameters. Dopant 40 is implanted ordiffused as an n-type or electrically insulating dopant in layers 31 and32 of mirror 24, preferably several layers above confining layer 18, tofunction in blocking current flow from the perimeter of active region 20and lower mirror 16, and to confine the current flow within dimension50. It is preferable for the depth of implant 40 to be several tenths ofa micron but may range between 0.1 and 2 microns. Dimension 50 may bebetween 0.1 and 60 microns, but is typically several microns, i.e., 2 to5 microns. Several more mirror periods (0 to 10) may be formed on top ofthe implanted or diffused surface followed by the mid-portion ofstructure 10, which consists of two Al_(x)Ga_((1−x))As (=0.6) confininglayers 18 and 22. The proportion indicated by x may be 0.25 or greater.There is p type doping in the layer nearest the p type mirror. Layers 18and 22 sandwich a region 20 having three GaAs quantum wells 28,separated from one another and confining layers 18 and 22 by fourAl_(x)Ga_((1−x))As (=0.25) barrier layers 36. The number of GaAs quantumwells may be from one to five. Alternatively, one could potentially havean active region 20 without quantum wells, e.g., a region having anemitting layer of about 0.2 micron thick. On top of confining layer 22on active region 20, a p-type mirror 24 is grown, consisting of 18periods of alternating layers of p-AlAs 31 and p-Al_(x)Ga_((1−x))As 32(=0.15 preferably, but may have any value greater than 0.05). The numberof periods may be greater or less than 18, depending on otherparameters. A GaAs contact layer 34 is formed on top of mirror 24. Aproton isolation implant 38 is placed at the perimeter of contact layer34, mirror 24, active region 20 and confining layer 22, to separate onedevice 10 from a like neighboring device on a chip. If a single laserchip 10 were to be made, then it is possible that one could eliminatethis proton implant 38, if the implant or diffusion made on top of then-mirror were to extend all the way to the edge of the chip. Laser 10connections are formed by depositing at least one p-type ohmic metalcontact 26 on the top surface of contact layer 34, and a broad arean-type ohmic contact 14 on the back side of wafer substrate 12. Theresulting device 10 emits laser light in the range of 760 to 870 nm.

Curves 62 and 63 of FIG. 8 reveal the concentration of dopant and Alcontent, respectively, in regions 18, 20 and 22.

What is claimed is:
 1. A VCSEL structure comprising: a lower mirrorcomprising a first dopant type; an upper mirror comprising a seconddopant type; an active gain region situated between said lower and uppermirrors; a first spacer situated between said lower mirror and saidactive gain region, having a portion adjacent to said active gain regionwhich is substantially undoped bv said first dopant type; and a secondspacer, comprising said second dopant type, situated between said uppermirror and said active gain region.
 2. The VCSEL structure of claim 1wherein a portion of said second spacer adjacent to said active gainregion is moderately doped with said second dopant type.
 3. The VCSELstructure of claim 2 wherein “moderately doped” means a doping levelgreater than 3e17.
 4. The VCSEL structure of claim 2 wherein “moderatelydoped” means a doping level greater than 6e17.
 5. The VCSEL structure ofclaim 2 wherein “moderately doped” means a doping level greater than1e18.
 6. The VCSEL structure of claim 3 wherein: said first spacer has acomposition of greater than 20 percent aluminum; and said second spacerhas a composition of greater than 20 percent aluminum.
 7. The VCSELstructure of claim 6 wherein said active gain region has a set ofalternating layers wherein every other layer has a composition of atleast 15 percent aluminum and a layer between the every other layer hasa composition of about zero percent aluminum.